3D Tiles demo has terrible imagery performance

In my machine, loading the 3D Tiles example in IE 11 and without ever moving the camera, it takes over a minute for all of the imagery to come in. If I use the same default view (below code snippet) but without 3D Tiles loaded, it only takes a couple of seconds. I'm not sure if this is a problem with the new request scheduler or something unrelated. I actually see a similar issue in Chrome but it's much faster (several seconds vs instant rather than a minute vs a few seconds).

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.camera.viewBoundingSphere({"center":{"x":1215020.301129722,"y":-4736341.9686844405,"z":4081630.311150717},"radius":200.90026973524507}, new Cesium.HeadingPitchRange(0, -2.0, 0));
viewer.camera.lookAtTransform(Cesium.Matrix4.IDENTITY);
```

Start with the 3D Tiles example and reproduce the default camera view from the issue in IE 11 and Chrome. Compare imagery loading with and without 3D Tiles, then inspect whether the request scheduler is involved. Done means the large performance gap is understood and the imagery loads acceptably in the reported scenario.

Written by the indexing model from the issue text.